The present invention relates to actively controlled suspension systems and in particular to processor based active suspension systems which control the power consumed by the suspension system.
Active suspension systems are known to those skilled in the art. In general, active suspension systems control a vehicle's "ride" and "handling". Active suspension systems control and maintain the vehicle's attitude, for example, elevation, roll and pitch. Suspension systems of this type sense the attitude of the car and, in response, provide fluid power to, or remove fluid power from, actuators to maintain and control the "ride" and "handling" of the vehicle. For example, Williams et al, U.S. Pat. No. 4,625,993, teaches a conventional active suspension system intended to maintain a vehicle substantially stable in all planes of movement irrespective of any external forces acting upon the vehicle.
In general, active suspension systems, for example in automobiles and the like, include a source of pressurized fluid. The fluid source, for example a hydraulic pump, provides fluid power in the form of a pressurized working fluid to suspension units, one at each vehicle wheel. The pressurized fluid source is typically driven by torque provided by the vehicle's engine.
The fluid power provided by the pump is critical to the performance of the active suspension system because ability to provide sufficient fluid power to dissipate and/or counteract undesirable external forces. The power demand upon the pump is therefore a function of the power requirements of the suspension units which are largely a function of the vehicle speed and the conditions of the road including its "texture". As the conditions at each suspension unit change, the fluid power demanded and consumed by each suspension unit changes.
The pump's ability to meet the suspension unit's power demand, however, impacts the overall performance characteristics of the vehicle. That is to say, the load imposed upon the engine by the suspension system's pump has a bearing on the performance and response of the vehicle. If the suspension system's pump rapidly reduces its load demand on the engine, then the vehicle may experience undesirable surge. If the suspension system's pump excessively or rapidly loads the engine, then the vehicle may experience undesirable hesitation. The rate at which power is being provided to the pump at any given time is dependent upon the current vehicle conditions, including, for example, the engine speed, the throttle position, the transmission gear position, the spark advance, and the vehicle velocity..
Active suspension systems, like the system disclosed in Fukunaga et al, U.S. Pat. No. 4,848,790, adjust the pump's power consumption based primarily upon the speed of the vehicle. The pressure of the working fluid provided to the suspension units by the pump is dependent upon the speed of the vehicle. When the speed of the vehicle is below a reference value the suspension system provides fluid communication between a first pump and the hydraulic cylinders. The first pump provides fluid at a first fluid pressure. When the speed of the vehicle is above the reference value, the system provides fluid communication between a second pump and the hydraulic cylinders. The second pump provides fluid at a second fluid pressure where the second pressure is greater than the first.
Fukunaga et al type suspension systems focus upon the pump's unnecessary power consumption and the resulting degradation of fuel economy and drivability of the vehicle including the ability of the vehicle to accelerate on demand. Active suspension systems of this type vary the power consumption by the pump in an effort to limit unnecessary power consumption and reduce degradation of fuel economy and drivability.
The Fukunaga et al type suspension systems, however, are neither concerned with nor address the effects of the rate of change of the pump power demand and/or consumption on the vehicle's performance. These systems do not monitor and control the rate of change of the pump's power demand and consumption.
There exists a need for an active suspension system that monitors the rate of change of the power demand and/or consumption by the system's pump and controls the rate of change when the rate of change of power demand or consumption exceeds a predetermined level which is dependent upon the current conditions of the vehicle. Further, there exists a need for a system that controls the rate of change of the pump's power demand and/or consumption to reduce the impact on the vehicle's performance characteristics and reduce undesirable hesitation and/or surge.